Unique hydro energy system harnesses slow water currents
November 25, 2008 Hydro-power systems are by far the most widely used form of renewable energy on the planet (in 2005 they accounted for an estimated 63% of all electricity produced via renewable sources), but despite their eco-friendly appeal the implementation of large scale facilities - particularly where dam building is involved - has some serious environmental and economic drawbacks. One solution to this impasse that is growing in support is to use leaner, less destructive systems like Bourne Energy's RiverStar to harness current flow and provide energy at a local level. This is the thinking behind VIVACE, a machine developed at the University of Michigan which applies the same principles fish use to swim efficiently in order to generate power from currents much slower than those required to drive designs based on turbines and water mills.
The majority of the Earth's ocean and river currents which are less than 3 knots. Most turbines and water mill systems need an average of 5 or 6 knots to operate efficiently, but VIVACE (which stands for Vortex Induced Vibrations for Aquatic Clean Energy) can make energy with less than 2 knots, so the potential is huge.
The system achieves this by harnessing what are known as "vortex induced vibrations". These are caused by the flow of liquid or air over a rounded or cylinder-shaped object. In this case a cylinder underwater is subjected to current and, without delving too far into fluid dynamics, as the liquid in contact with its surface "sticks" and slows down around it to create vortices or eddies on opposite sides of the cylinder. Alternating vortices form above and below the cylinder, exerting force perpendicular to the current. Attach the cylinder to springs so that it oscillates and you have a renewable source of mechanical energy which can be used to produce electricity.
This phenomena is actually very destructive when it comes to building structures underwater or into the air, causing fatigue damage of offshore oil rigs and even toppling the Tacoma Narrows bridge in Washington in 1940.
"For the past 25 years, engineers—myself included—have been trying to suppress vortex induced vibrations. But now at Michigan we're doing the opposite. We enhance the vibrations and harness this powerful and destructive force in nature," said VIVACE developer Michael Bernitsas, a professor in the U-M Department of Naval Architecture and Marine Engineering.
Fish on the other hand, use vortex induced vibrations to their advantage.
"VIVACE copies aspects of fish technology," Bernitsas said. "Fish curve their bodies to glide between the vortices shed by the bodies of the fish in front of them. Their muscle power alone could not propel them through the water at the speed they go, so they ride in each other's wake."
The prototype machine uses a cylinder hanging horizontally across the flow of water in large tank tank in which water in the tank flows at 1.5 knots and at this stage, it looks nothing like a fish, but Bernitsas' says future versions will have the equivalent of a tail and surface roughness a kin to scales.
Just a few cylinders might be enough to power an anchored ship, or a lighthouse according to Bernitsas and an array of VIVACE converters the size of a running track and about two stories high could power about 100,000 houses and produce energy at 5.5 cents per kilowatt hour.
"There won't be one solution for the world's energy needs," Bernitsas said. "But if we could harness 0.1 percent of the energy in the ocean, we could support the energy needs of 15 billion people."
The system would also be kind to the creatures that helped inspire it as the slowly oscillating VIVACE would in theory, pose no threat to marine life.
A feasibility study in the Detroit River was recently completed and researchers are working to deploy a pilot project within the 18 months.
The technology is being commercialized through Bernitsas' company, Vortex Hydro Energy and a paper on VIVACE is published in the current issue of the quarterly Journal of Offshore Mechanics and Arctic Engineering.
Source: University of Michigan